Feb. 25, 2011 -- A simple blood test that measures long-term glucose levels -- the hemoglobin A1c (HbA1c) test -- may not be the best way to diagnose diabetes in adolescents.

The HbA1c screening test is easier to perform than the fasting plasma glucose test, which requires fasting for eight hours. But the new study, which appears in the Journal of Pediatrics, shows that the HbA1c test is less sensitive in diagnosing diabetes and prediabetes in children than in adults.

"My worry is that we could miss cases of diabetes," says study researcher Joyce Lee, MD, a pediatric endocrinologist at University of Michigan's Mott Children's Hospital in Ann Arbor. "The HbA1c test just doesn't perform as well in kids as it does in adults."Lee says this test can be useful as an adjunct to other diabetes tests.

"You don't want to rely on just this test to diagnose a child with diabetes," she says. If a child is overweight and obese and has two of four risk factors for diabetes, they should be screened in some other way beside HbA1c."

Risk factors include family history of diabetes, maternal history of gestational diabetes, belonging to certain ethnic groups (Native Americans, African-Americans, or Asians/South Pacific Islanders), or any signs of insulin resistance such as polycystic ovary syndrome, high blood pressure, or high cholesterol levels.

Comparing Diabetes Screening Tests

In the new study, researchers compared HbA1c screening results with those seen on fasting glucose tests among 1,156 obese and overweight adolescents aged 12 to 18. They compared these readings with those of 6,751 adults aged 19 to 79.

According to the American Diabetic Association guidelines, diabetes is diagnosed when an HbA1c level is 6.5% or more; prediabetes is diagnosed when an HbA1C level is between 6% and 6.4% on two separate tests. Prediabetes is marked by higher than normal glucose levels that places a person at risk for developing type 2 diabetes.

Using these cut-offs, the fasting test caught significantly more adolescents with diabetes and prediabetes than the HbA1c test, the study showed. The HbA1c was much more sensitive among adults than adolescents.

The researchers also compared results in a subset of 267 adolescents and 1,476 adults who had two-hour plasma glucose measurements, in which blood glucose is measured exactly two hours after you have a sweet drink. The two-hour test was also more sensitive than the HbA1c test in adolescents.

"Most of the studies that were done to validate the HbA1c were done in adults," Lee says. There are hormonal differences in children that may affect its ability to diagnose diabetes, she says. "There is also an age-dependent rise in HbA1c over time, so we many need a lower threshold."

Gerald Bernstein, MD, director of the diabetes management program at the Friedman Diabetes Institute of Beth Israel Medical Center in New York City, agrees that the HbA1c test results can be misleading in children. He says that changing hormone levels in boys and girls may skew the results.

"There is a lot of data that needs to be accumulated and validated in the technical sense to be able to use this test as reliable diagnostic criteria," he says.

"If your child is at risk for diabetes, I would not be happy with just this test unless it was high," he says. "If it was high, you escape the ambiguousness seen in the new study."